Pressure sensor

Abstract

The invention provides a pressure sensor which is simple in structure and wide in pressure detection range. A sensor element (20) of a pressure sensor (10) is provided with: a diaphragm (21) provided so as to be displaceable by receiving the pressure of a fluid (F); a pressure-sensitive capacitor (C1) and a reference capacitor (C2) that detect the amount of displacement of the diaphragm (21); and a piezoelectric layer (26) that displaces the diaphragm (21). Furthermore, on the basis of the amount of deformation of the diaphragm (21) detected by the pressure-sensitive capacitor (C1) and the reference capacitor (C2), the control circuit (30) applies, to the piezoelectric layer (26), a drive voltage for maintaining the diaphragm (21) at a prescribed position such as a zero point, and outputs, as a signal indicating the pressure (Pf) of the fluid (F), a signal indicating the drive voltage applied to the piezoelectric layer (26).

Description

TECHNICAL FIELD

[0001] The present application relates to a pressure sensor. BACKGROUND

[0002] A pressure sensor using a diaphragm (for example, a diaphragm vacuum gauge) is capable of pressure detection without gas species dependency, and is therefore used for a wide range of applications such as a gas supply system, a process system, an exhaust system, and the like in a semiconductor manufacturing apparatus (Patent Literature 1).

[0003] Further, in order to achieve multiple measurement purposes such as chamber opening detection in a manufacturing process or process control, sometimes multiple diaphragm vacuum gauges having different pressure ranges (pressure detection ranges) are provided. That is, there is no diaphragm vacuum gauge capable of measuring the entire pressure range with one unit, because of the structural limit of a sensor that uses a diaphragm to bear pressure. More specifically, for example, in order to avoid damage when an excessive pressure is applied, the stress generated in the diaphragm should of course be below the breaking stress, but in terms of ensuring the output accuracy of the sensor, the diaphragm must be within the range of elastic deformation, and the output sensitivity of the sensor must be ensured, and for these reasons, the movable range of the diaphragm is limited. Therefore, in practice, sensors corresponding to each pressure detection range are used in combination. In a case where multiple sensors are configured for each corresponding pressure detection range as such, for example, a diaphragm vacuum gauge occupies a large amount of space within a semiconductor manufacturing apparatus. Further, the connection portions of the piping are also increased, and therefore the risk of leakage of toxic material gas flowing in the piping is also increased.

[0004] Therefore, as a pressure sensor capable of realizing a wider pressure detection range than in the past, there is a pressure sensor that amplifies an output signal by an arithmetic circuit, or that uses multiple sensor elements housed in the same frame (Patent Literature 2). However, in this case, a large amount of space is occupied.

[0005] Further, as a pressure sensor, there is a force balance type sensor that is driven by electrostatic attraction corresponding to the displacement of a diaphragm to maintain the diaphragm at an initial position (Patent Literature 3).

[0006] [Related Art Documents] [Patent Literature] Patent Literature 1: Japanese Patent Application Laid-Open No. 2010-236949 Patent Literature 2: Japanese Patent Application Laid-Open No. H6-102128 Patent Literature 3: Japanese Patent Application Laid-Open No. 2001-124643 SUMMARY [Problems to be Solved by the Invention] In a diaphragm type pressure sensor using a diaphragm, a pressure detection range is limited by a movable range of the diaphragm. Therefore, in a case where a diaphragm is driven by electrostatic attraction, since the diaphragm is driven by the attraction, if displacement (flexion) of the diaphragm at a low pressure such as a diaphragm vacuum gauge is assumed, a structure including a configuration for giving the diaphragm a side of a pressure receiving surface with the electrostatic attraction is included. In this case, for example, a configuration element or the like for protecting an electrode generating the electrostatic attraction from an etching material included in a measurement target gas in a semiconductor manufacturing device is required, which is a cause of complicating the sensor structure. For these reasons, a diaphragm type pressure sensor which is simple in structure and wide in a measurable pressure range (pressure detection range) and which reduces configuration elements to be provided on a side of a pressure receiving surface of the diaphragm is required.

[0007] An object of the present application is to provide a pressure sensor which is simple in structure and wide in pressure detection range.

[0008] [Technical means for solving the problem] The pressure sensor of the present application includes: a diaphragm which is provided so as to be displaceable by a pressure of a fluid; a detection element which detects an amount of displacement of the diaphragm; a piezoelectric layer which displaces the diaphragm; and a control circuit which, based on the amount of displacement detected by the detection element, applies a drive voltage which maintains a position of the diaphragm at a prescribed position to the piezoelectric layer, and outputs a signal indicating the drive voltage applied to the piezoelectric layer as a signal indicating the pressure of the fluid.

[0009] [Effects of the invention] According to the present application, it is possible to provide a pressure sensor which is simple in structure and wide in pressure detection range. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 is a structural diagram of a pressure sensor of an embodiment of the present application, including a cross-sectional view of a sensor element.

[0011] Figure 2 is a closed loop block diagram of a pressure sensor of an embodiment of the present application.

[0012] Figure 3 is a circuit structural diagram of a pressure sensor of a modification example.

[0013] Figure 4 is a cross-sectional view of a sensor element of a modification example.

[0014] Figure 5 is a cross-sectional view of a sensor element of a modification example.

[0015] EXPLANATION OF REFERENCE NUMERALS 10: pressure sensor 20, 120, 220: sensor element 21: diaphragm 22: support portion 22A: lower surface 23: facing portion 24A to 24E, 124B, 124BA to 124BC: electrode 25A to 25E: wiring 26, 126, 226: piezoelectric layer 126A to 126C, 224B, 224D, 224P, 224Q: piezoelectric film 30: control circuit 31: summing point 32: amplifier 131: oscillation circuit 132: RC circuit 133: RC circuit 134: comparator 135: RC circuit 136: voltage boosting circuit C1: piezoresistor C2: reference capacitor C: piezoresistor capacity Cref: reference capacity F: fluid Fa: force Fp: force N: correction value Pf: pressure R1: resistance R2: resistance S1: space S2: reference pressure chamber DETAILED DESCRIPTION Hereinafter, an embodiment of the present application and its modification will be described with reference to the drawings.

[0016] As Figure 1 illustrated, a pressure sensor 10 is configured to include a sensor element 20 and a control circuit 30 that detects a pressure Pf of a fluid (measured fluid) F through the sensor element 20. The pressure sensor 10 also includes a not-illustrated frame or the like that houses the sensor element 20 and the control circuit 30. In addition, Figure 1 the up-down and left-right directions are set for convenience of explanation and do not limit the mounting direction of the pressure sensor 10. As the fluid F, for example, a vacuum pressure medium or a rarefied fluid or the like can be cited. Moreover, Figure 1 here, the sensor element 20 is represented in a sectional view, but the sectional view appropriately omits the illustration of the inside structure of the sectioned surface (the structure of the side of the sectioned surface Figure 4 and Figure 5 as well).

[0017] The sensor element 20 includes a diaphragm 21, a support portion 22, an opposing portion 23, electrodes 24A to 24E, wiring lines 25A to 25E, and a piezoelectric layer 26.

[0018] The diaphragm 21 receives the pressure Pf of the fluid F introduced into the frame of the pressure sensor 10. The diaphragm 21 is displaced by the received pressure Pf. As a material of the diaphragm 21, sapphire can be cited, but the material is not limited thereto. For example, aluminum nitride (AIN), aluminum oxide (AI2O3), yttrium aluminum garnet (YAG), yttrium oxide (Y2O3), silicon carbide (SiC), and magnesium oxide (MgO) can be cited as examples of the material.

[0019] The support portion 22 supports the diaphragm 21. The support portion 22 is, for example, a quadrangular shape in plan view (when viewed from above), and the diaphragm is a circular shape. The support portion 22 is integrally formed of the same material as the diaphragm 21. As a modification, the support portion 22 can be formed of a member independent of the diaphragm 21. For example, the support portion 22 includes one or a plurality of members that fix the edges of a sheet-like member that constitutes the diaphragm 21. In this case, the edges of the sheet-like member, that is, the portions that do not displace due to the pressure Pf, become a part of the support portion 22, and the central portion of the sheet-like member, that is, the portion that displaces due to the pressure Pf, becomes the diaphragm 21.

[0020] The inner surface of the upper portion of the support portion 22 and the upper surface of the diaphragm 21 form a space SI that introduces the fluid F to be introduced into the frame of the pressure sensor 10. The upper surface of the diaphragm 21 becomes a pressure receiving surface that receives the pressure Pf of the fluid F introduced into the space SI.

[0021] The opposing portion 23 is fixed to the lower portion of the support portion 22 and opposes the diaphragm 21. The reference pressure chamber S2 is constituted by the inner surface of the support portion 22, the lower surface of the diaphragm 21, and the upper surface of the opposing portion 23.

[0022] In the reference pressure chamber S2, a reference pressure that is compared with the pressure Pf of the fluid F is introduced. The pressure Pf is not introduced into the reference pressure chamber S2. The reference pressure is a vacuum pressure, a pressure of a prescribed gas (for example, an inert gas) sealed into the reference pressure chamber S2, or the like. The lower surface of the diaphragm 21 becomes a reference pressure receiving surface that receives the reference pressure. The diaphragm 21 is displaced in correspondence with the difference between the pressure Pf of the fluid F received by the upper surface and the reference pressure of the reference pressure chamber S2 received by the lower surface. Generally, since the former pressure is large, the diaphragm 21 is displaced in a manner that is concave downward (in the direction of the opposing portion 23).

[0023] The electrodes 24A to 24E are formed in the reference pressure chamber S2 together with the piezoelectric layer 26. The electrodes 24A to 24E and the piezoelectric layer 26 are each formed in a film shape. The electrodes 24A to 24E are formed of an arbitrary conductive material described later together with the wirings 25A to 25E. The piezoelectric layer 26 includes one or a plurality of piezoelectric films formed of a piezoelectric material such as aluminum nitride (AIN).

[0024] The electrode 24A is formed on the lower surface of the diaphragm 21. The piezoelectric layer 26 is formed on the lower surface of the electrode 24A. The electrode 24B is formed on the lower surface of the piezoelectric layer 26. The piezoelectric layer 26 is sandwiched by the electrodes 24A and 24B. The electrode 24C is formed, for example, on the lower surface 22A of the support portion 22 around the diaphragm 21 and the lower surface 22A facing the facing portion 23 at intervals. The electrode 24C can also be arranged so as to be biased toward the support portion 22 of the lower surface of the diaphragm 21.

[0025] The electrodes 24D and 24E are formed on the upper surface of the facing portion 23. The electrode 24D faces the electrode 24B at intervals. The electrode 24E faces the electrode 24C at intervals.

[0026] The electrodes 24A, 24B, and the piezoelectric layer 26 displace in accordance with the displacement of the diaphragm 21 caused by the pressure Pf of the fluid F. On the other hand, the electrode 24C is arranged at a position deviated from the diaphragm 21, and the displacement of the diaphragm 21 is small even if the diaphragm 21 displaces. The electrodes 24B and 24D constitute a piezoresistive capacitor Cl in which the electrode 24B is provided as a movable electrode and the electrode 24D is provided as a fixed electrode. The electrodes 24C and 24E constitute a reference capacitor C2 having a reference capacity Cref which is an electrostatic capacity compared with the electrostatic capacity of the piezoresistive capacitor Cl, that is, a piezoresistive capacity C. The electrodes 24B to 24E are formed so that the piezoresistive capacity C = the reference capacity Cref when the diaphragm 21 is flat (the areas of the electrodes and the distances between the electrodes are adjusted). The state in which the diaphragm 21 is flat is a state in which the pressure Pf of the fluid F and the reference pressure of the reference pressure chamber S2 are in force balance, and can be said to be a state in which the diaphragm 21 is at a zero point position.

[0027] The wirings 25A to 25E are connected to the control circuit 30 through the facing portion 23.

[0028] The electrodes 24A to 24C are formed on either or both of the lower surface 22A of the support portion 22 and the lower surface of the diaphragm 21 by a prescribed film formation patterning together with a part of the piezoelectric layer 26 and the wiring 25A to 25C. The electrodes 24D and 24E are formed on the upper surface of the facing portion 23 by a prescribed film formation patterning. A part of the wiring 25D and 25E can also be patterned together with the electrodes 24D and 24E.

[0029] The control circuit 30 inputs a measurement potential for driving the capacitors C1 and C2, that is, for measuring the pressure-sensitive capacitance C and the reference capacitance Cref, to the electrodes 24B and 24C. The control circuit 30 has a charge amplifier connected to the electrodes 24D and 24E via the wiring 25D and 25E, respectively. The charge amplifier converts a flowing current into a voltage, thereby outputting a voltage signal proportional to each of the capacitance values C and Cref. The control circuit 30 can also be configured to input the measurement potential to the electrodes 24D and 24E. At this time, the charge amplifier is connected to the electrodes 24B and 24C. Further, the control circuit 30 inputs a drive voltage for driving the piezoelectric layer 26 between the electrodes 24A and 24B. The control circuit 30 inputs a potential corresponding to the potential of the electrode 24B to the electrode 24A so that the potential difference between the electrodes 24A and 24B becomes the drive voltage.

[0030] The pressure Pf of the fluid F displaces the diaphragm 21. This displacement changes the pressure-sensitive capacitance C of the pressure-sensitive capacitor C1. The amount of change in the pressure-sensitive capacitance C, that is, the amount of displacement of the diaphragm 21, is represented by the difference between the pressure-sensitive capacitance C and the reference capacitance Cref of the reference capacitor C2. The control circuit 30 applies a drive voltage corresponding to the amount of change in the pressure-sensitive capacitance C, that is, the amount of displacement of the diaphragm 21, to the piezoelectric layer 26 via the electrodes 24A and 24B. The piezoelectric layer 26, for example, stretches and contracts in a planar direction perpendicular to the thickness direction (the up-down direction) by the application of the drive voltage from the thickness direction. Specifically, the piezoelectric layer 26 stretches and contracts in the planar direction (contracts by the rise of the drive voltage and expands by the fall of the drive voltage, for example) in accordance with the magnitude of the drive voltage from the thickness direction. The diaphragm 21 is displaced by the stretching and contracting. The control circuit 30 controls the drive voltage, thereby maintaining the diaphragm 21 at a zero point position. The greater the pressure Pf of the fluid F, the greater the amount of displacement of the diaphragm 21 due to the pressure Pf, as a result of which the drive voltage also becomes greater. That is, the drive voltage represents the pressure Pf of the fluid F. The control circuit 30 detects the pressure of the fluid by obtaining the drive voltage. The control circuit 30 outputs a signal representing the drive voltage to the outside as a detection signal for indicating the pressure Pf of the fluid F to the user.

[0031] As an example, the control circuit 30 is configured to realize a closed loop block diagram shown in FIG. 8 in conjunction with the sensor element 20. Figure 2 The control circuit 30 is configured to include a summing point 31 and an amplifier 32.

[0032] The force Fp by which the diaphragm 21 is displaced by the pressure Pf of the fluid F is subtracted from the force Fa by which the diaphragm 21 is displaced by the piezoelectric layer 26. The subtracted force AF displaces the diaphragm 21 if the value thereof is not 0. The force AF is reflected to the piezoelectric capacity C of the piezoelectric capacitor C1.

[0033] The control circuit 30 subtracts, at the summing point 31, the reference capacity Cref of the reference capacitor C2 (specifically, a voltage signal outputted from a charge amplifier connected to the electrode 24E) from the piezoelectric capacity C (specifically, a voltage signal outputted from a charge amplifier connected to the electrode 24D). In a case where a noise component is included in at least one of the piezoelectric capacity C and the reference capacity Cref, the control circuit 30 can also add, at the summing point 31, a correction value N by which the noise is subtracted to the difference between the piezoelectric capacity C and the reference capacity Cref.

[0034] The control circuit 30 amplifies, by the amplifier 32, a signal indicating the difference between the piezoelectric capacity C and the reference capacity Cref. The control circuit 30 applies, as a drive voltage, the signal amplified by the amplifier 32 to the piezoelectric layer 26 (the electrode 24A and the electrode 24B). Also, the control circuit 30 outputs, as a detection signal indicating the pressure Pf of the fluid F, the signal to the outside. The application of the drive voltage and the output of the detection signal to the outside continue until the amplification and the output of the signal indicating the difference between the piezoelectric capacity C and the reference capacity Cref next performed. The amplification of the signal indicating the difference between the piezoelectric capacity C and the reference capacity Cref includes a process of amplifying the signal this time and adding it to the signal amplified last time.

[0035] The control circuit 30 can also input, to the electrode 24D and the electrode 24E, a pulse signal for converting the capacities C and Cref of the capacitor C1 and the capacitor C2 to voltage signals. In this case, the control circuit 30 includes, for example, a circuit shown in FIG. 9. Hereinafter, the description of the overlapping parts is omitted while focusing on the parts different from the embodiment. Figure 3

[0036] The control circuit 30 includes an oscillation circuit 131, resistors R1 and R2, a comparator 134, a resistor-capacitance (RC) circuit 135, the amplifier 32, and a step-up circuit 136.

[0037] ​The oscillation circuit 131 oscillates to generate a pulse signal. The resistor Rl and the resistor R2 are connected in parallel to the oscillation circuit 131. The resistor Rl and the pressure-sensitive capacitor Cl constitute an RC circuit 132. The resistor R2 and the reference capacitor C2 constitute an RC circuit 133. The pulse signal from the oscillation circuit 131 is input to each of the RC circuit 132 and the RC circuit 133.

[0038] The RC circuit 132 converts the waveform of the pulse signal from the oscillation circuit 131 into a waveform corresponding to the pressure-sensitive capacity C. The pressure-sensitive capacity C varies depending on the ΔF of the diaphragm 21, and therefore the signal waveform converted by the RC circuit 132 also varies due to the displacement of the diaphragm 21. The RC circuit 133 converts the waveform of the pulse signal from the oscillation circuit 131 into a waveform corresponding to the reference capacity Cref. Since the reference capacity Cref is substantially constant, the waveform of the signal is also substantially constant.

[0039] The comparator 134 takes the difference between the outputs from the RC circuit 132 and the RC circuit 133. The RC circuit 135 smoothes the output of the comparator 134. The output of the comparator 134 that has been smoothed, that is, the output of the RC circuit 135 indicates the difference between the pressure-sensitive capacity C and the reference capacity Cref, that is, the amount of change in the pressure-sensitive capacity C due to the displacement of the diaphragm 21 from the zero position. The output of the RC circuit 135 is supplied to the amplifier 32 and amplified (details are described above). The voltage of the output amplified by the amplifier 32 is further boosted by the voltage boosting circuit 136 to the voltage level at which the piezoelectric layer 26 can be driven, and, as with the above, applied to the piezoelectric layer 26 (the electrode 24A and the electrode 24B) as a drive voltage. Also, the control circuit 30 outputs the output signal from the amplifier 32 (which can be said to be a signal indicating the drive voltage applied to the piezoelectric layer 26) to the outside as a detection signal indicating the pressure Pf of the fluid F.

[0040] As described above, according to the present embodiment and modifications, the sensor element 20 of the pressure sensor 10 includes: the diaphragm 21 that is displaceably provided by receiving the pressure Pf of the fluid F; the pressure-sensitive capacitor Cl and the reference capacitor C2 that detect the displacement amount of the diaphragm 21; and the piezoelectric layer 26 that displaces the diaphragm 21. Further, the control circuit 30 applies, to the piezoelectric layer 26, a drive voltage that maintains the diaphragm 21 at the zero point position, on the basis of the displacement amount of the diaphragm 21 detected by the pressure-sensitive capacitor Cl and the reference capacitor C2, and outputs a signal that represents the drive voltage applied to the piezoelectric layer 26 as a signal that represents the pressure Pf of the fluid F. With such a structure, even if the pressure Pf of the fluid F becomes large, the diaphragm 21 is maintained at the zero point position, and thus the detection range of the pressure Pf becomes wide, not depending on the movable range of the diaphragm (for example, the range in which the electrode 24B does not land on the electrode 24D). Further, by force-balance-controlling the diaphragm 21 at the zero point position on the basis of the displacement amount of the diaphragm 21, a high-precision pressure detection can be achieved, as compared with the conventional pressure sensor that uses an open loop to detect the pressure. Note that the drive voltage is only required to be a drive voltage that maintains the diaphragm 21 at a prescribed position. Here, the prescribed position is the zero point position, but can be any other position.

[0041] Further, the piezoelectric layer 26 is used for the position maintenance of the diaphragm 21, and the constituent elements that are to be provided on the pressure receiving surface side can be reduced, and thus the structure of the sensor element 20 becomes simple. Further, the drive voltage that drives the piezoelectric layer 26 is applied in the film thickness direction of the piezoelectric layer 26, and is not applied between the capacitor electrodes of the pressure sensor. Thus, problems caused by static electricity, such as landing of the diaphragm 21 toward the facing portion 23, or the like, due to discharge or pull in between the capacitor electrodes, are excluded. In this sense, too, the structure can be made simple without a countermeasure structure or the like against the problems caused by static electricity, and thus a sensor with high reliability, which has low-temperature sensitivity and corrosion resistance, and is less affected by external disturbances, can be designed.

[0042] Further, the piezoelectric layer 26 is preferably formed of the same material as the diaphragm 21, or a material that has a thermal expansion coefficient equivalent to (including approximately equivalent to) that of the diaphragm 21. For example, the material of the piezoelectric layer 26 and the diaphragm 21 is aluminum nitride (AlN). With such a structure, when the pressure sensor 10 as a whole is heated, the generation of warping due to the difference in the thermal expansion rates of the piezoelectric layer 26 and the diaphragm 21 is suppressed, and the output error of the pressure sensor 10 becomes small.

[0043] In this embodiment, one of the pair of electrodes 26A and 26B that applies a drive voltage to the piezoelectric layer 26 constitutes a piezoelectric capacitor Cl, but as a modification, the pair of electrodes that constitutes the piezoelectric capacitor and the pair of electrodes that apply a drive voltage to the piezoelectric layer with the piezoelectric layer interposed therebetween can be independently provided. Details of the modification will be described later as Modification 2. According to the configuration of this embodiment, the total number of electrodes can be reduced compared to the modification.

[0044] As a detection element that detects the displacement amount of the diaphragm 21, a piezoelectric capacitor Cl and a reference capacitor C2 are employed in this embodiment. Thereby, the displacement amount of the diaphragm 21 can be captured with good accuracy. In particular, noise resistance and the like can be obtained. In a case where the relationship between the electrostatic capacity of the piezoelectric capacitor Cl and the displacement amount of the diaphragm 21 has been uniquely determined, and / or in a case where the signal noise of the sensor is sufficiently small, the reference capacitor C2 can also be omitted.

[0045] The group of the electrode 24B and the electrode 24C and the group of the electrode 24D and the electrode 24E to which the measurement potential is input can also be formed as one electrode. Thereby, the electrode shape can be simplified.

[0046] (Modification 1) The piezoelectric layer 26 can contain one piezoelectric film as shown in Figure 1 , or can contain a plurality of piezoelectric films as in the modification shown in Figure 4 . In the former type, the piezoelectric film can be increased in size, and thus the entire diaphragm 21 can be displaced, and thus a larger displacement amount of the diaphragm 21 can be obtained. In the latter, the displacement amount (drive voltage) is controlled by each of the plurality of piezoelectric films, and thus the displacement of the diaphragm 21 can be precisely controlled, and the influence of initial warping and the like in a case where the diaphragm 21 is thin can be reduced. Furthermore, in a case where both positive and negative voltages are used, the displacement of the diaphragm 21 in the upward direction in the drawing can also be controlled. Figure 1

[0047] Figure 4 In the sensor element 120 of the modification shown in, a piezoelectric layer 126 containing piezoelectric films 126A to 126C is provided instead of the piezoelectric layer 26, and an electrode 124B formed instead of the electrode 24B. The piezoelectric film 126A is provided at the center of the diaphragm 21, and the piezoelectric films 126B and 126C are formed in a circular arc shape or the like so as to surround the diaphragm 21. The piezoelectric films that surround the diaphragm 21 can also be provided as three or more piezoelectric films. Conversely, the piezoelectric films can be provided as one body in a circular ring plate shape. The electrode 124B contains electrodes 124BA to 124BC that are respectively provided on the lower surfaces of the piezoelectric films 126A to 126C. Figure 4 In this embodiment, the illustration of the wiring and the illustration of the control section 30 (the control section 30 will be described later Figure 5 ) are omitted. The wiring can be routed in any manner (the wiring will be described later ).Figure 5 The same applies to China.

[0048] In this modified example, electrode 124BA and electrode 24D constitute the varistor C1. Control circuit 30 ( Figure 1 For example, a charge amplifier connected to electrodes 24D and 24E inputs a measurement potential to electrodes 124BA to 124BC and electrode 24C. For instance, the control circuit 30 inputs a first potential (the potential of the driving voltage constituting piezoelectric film 126A) to electrode 124BA corresponding to the displacement of the diaphragm 21 (the difference between the piezoelectric capacitance C and the reference capacitance Cref), and a second potential (the potential of the driving voltage constituting piezoelectric films 126B and 126C) to electrodes 124BB and 124BC. By separately controlling the first and second potentials, the displacement of the diaphragm 21 can be precisely controlled. Alternatively, at least one of electrodes 124BB and 124BC, whose displacement is relatively small even when the diaphragm 21 is displaced, can be designated as an electrode of the reference capacitor C2. In this case, the other electrode of the reference capacitor C2 is provided in the portion facing at least one of electrodes 124BB and 124BC. Furthermore, electrode 24D can be extended to a position facing at least one of electrodes 124BB and 124BC. Also, piezoelectric layers 126A and 124BA in the figure can be omitted, and only the varistor capacitor C1 can be provided in the center. Moreover, when a driving voltage is applied to electrodes 24A and 124B, a potential difference sufficient to achieve a specified displacement is obtained in piezoelectric layers 126A to 126C.

[0049] (Variation Example 2) like Figure 5 As shown, the sensor element 220 of the pressure sensor in this modified example includes: a pair of electrodes 224B and 224D constituting a piezoresistive capacitor C1; and a pair of electrodes 224P and 224Q, which sandwich the piezoelectric layer 226 (replacing the piezoelectric layer 26) from above and below, thereby applying a driving voltage to the piezoelectric layer 226. The pair of electrodes 224B and 224D are formed as separate entities and spaced apart from the pair of electrodes 224P and 224Q. Electrodes 224B and 224D are formed, for example, in a circular plate shape. The combination of the piezoelectric layer 226 and the pair of electrodes 224P and 224Q is formed, for example, in an annular plate shape surrounding the outer periphery of electrodes 224B and 224D. The combination may also have multiple portions spaced apart from each other, such as the piezoelectric layer 126 in modified example 1. In this case, each of the multiple portions includes a piezoelectric film as part of the piezoelectric layer 226 and a pair of electrode films as part of each of the pair of electrodes 224P and 224Q. Each of the multiple parts is formed, for example, into an arcuate plate shape by dividing a circular annular plate shape. At least one of a pair of electrode films of each part (e.g., an electrode film connected to a reference potential) may also be continuously connected.

[0050] Control circuit 30 ( Figure 1 A voltage for capacitance measurement is applied to a pair of electrodes 224B and 224D constituting the varistor capacitor C1 and a pair of electrodes 24C and 24E constituting the reference capacitor C2. Control circuit 30 ( Figure 1 A driving voltage is applied to a pair of electrodes 224P and 224Q to deform the piezoelectric layer 226. As described above, when the combination of the piezoelectric layer 226 and the pair of electrodes 224P and 224Q comprises multiple parts spaced apart from each other, the control circuit 30 ( Figure 1 A driving voltage is applied to each part. The driving voltage can be common to each part or different to each part (see also the description of Modified Example 1). According to this modified example, the measuring voltage for the varistor C1 and the driving voltage for the piezoelectric layer 26 can be controlled separately, thus reducing the burden of voltage control performed by the control circuit 30. Moreover, the pair of electrodes 224B and 224D constituting the varistor C1 are less susceptible to electrical influence from the piezoelectric layer 126, so the displacement of the diaphragm 21 is more accurately reflected in the varistor capacitance C of the varistor C1.

[0051] (Variation Example 3) The diameter of the diaphragm 21 can be the same as, shorter than, or longer than the diameter of the reference pressure chamber S2. The diameter of the piezoelectric layer 26 (especially a piezoelectric film formed in the same shape as the diaphragm 21) can be the same as, shorter than, or longer than the diameter of the diaphragm 21. The electrode between the piezoelectric layer 26 (especially a piezoelectric film formed in the same shape as the diaphragm 21) and the diaphragm 21 can be a single piece or divided into multiple parts.

[0052] (Scope of the invention) The present invention has been described above with reference to embodiments and modifications, but the present invention is not limited to the described embodiments and modifications. For example, the present invention includes various modifications to the described embodiments and modifications that can be understood by those skilled in the art within the scope of the technical concept of the present invention. For example, the control circuit 30 may also include a combination of at least one or more of a computer, a field-programmable gate array (FPGA), and an application-specific integrated circuit (ASIC) that executes the program to perform the processing. The various structures listed in the embodiments and modifications can be appropriately combined within a non-contradictory scope. Moreover, any structure among the structures listed above may be omitted.

[0053] (Note) The following notes describe the structure of at least a part of the present embodiment and modifications. The present application is not limited to the following notes.

[0054] (Note 1) A pressure sensor comprising: a diaphragm configured to be displaced by a pressure of a fluid; a detection element configured to detect an amount of displacement of the diaphragm; a piezoelectric layer configured to displace the diaphragm; and a control circuit configured to apply, to the piezoelectric layer, a drive voltage that maintains a position of the diaphragm at a prescribed position, based on the amount of displacement detected by the detection element, and output a signal indicative of the drive voltage applied to the piezoelectric layer as a signal indicative of the pressure of the fluid.

[0055] (Note 2) The pressure sensor according to Note 1, wherein the piezoelectric layer includes one or more piezoelectric films.

[0056] (Note 3) The pressure sensor according to Note 1 or 2, further comprising: a pair of electrodes configured to apply the drive voltage to the piezoelectric layer, the diaphragm has a pressure receiving surface configured to receive the pressure of the fluid and a reference pressure surface configured to receive a pressure that is compared with the pressure, the piezoelectric layer and the pair of electrodes are layered on the reference pressure surface, the detection element includes a pressure-sensitive capacitor whose electrostatic capacity changes in correspondence with the amount of displacement of the diaphragm, and the pressure-sensitive capacitor includes a fixed electrode provided at an opposing portion that opposes the diaphragm in the thickness direction, and the electrode on the side of the opposing portion among the pair of electrodes is configured as a movable electrode.

[0057] (Note 4) The pressure sensor according to any one of Notes 1 to 3, wherein the diaphragm includes sapphire, aluminum nitride (AlN), aluminum oxide (Al2O3), yttrium aluminum garnet (YAG), yttrium oxide (Y2O3), silicon carbide (SiC), or magnesium oxide (MgO).

[0058] (Note 5) The pressure sensor according to any one of Notes 1 to 4, wherein the diaphragm and the piezoelectric layer are made of the same material or have the same coefficient of thermal expansion.

[0059] (Note 6) The pressure sensor according to any one of the above notes 1 to 5, further comprising: a pair of electrodes that apply the drive voltage to the thickness direction of the piezoelectric layer, the piezoelectric layer expands and contracts in a direction perpendicular to the thickness direction when the drive voltage is applied to the thickness direction.

[0060] (Note 7) The pressure sensor according to any one of the above notes 1 to 6, wherein the detection element includes a piezo-capacitor whose electrostatic capacity changes in accordance with the displacement amount of the diaphragm, and a reference capacitor having a reference capacity that is compared with the electrostatic capacity of the piezo-capacitor, the control circuit detects the displacement amount based on the electrostatic capacity and the reference capacity.

[0061] (Note 8) The pressure sensor according to any one of the above notes 1 to 7, further comprising: a pair of electrodes that apply the drive voltage to the piezoelectric layer, the detection element includes a piezo-capacitor whose electrostatic capacity changes in accordance with the displacement amount of the diaphragm, one of the pair of electrodes constitutes the piezo-capacitor.

[0062] (Note 9) The pressure sensor according to any one of the above notes 1 to 8, further comprising: a pair of electrodes that apply the drive voltage to the piezoelectric layer, the detection element includes a piezo-capacitor whose electrostatic capacity changes in accordance with the displacement amount of the diaphragm, two electrodes that constitute the piezo-capacitor are provided independently of the pair of electrodes.

Claims

1. A pressure sensor comprising: a diaphragm which is provided so as to be displaceable by a pressure of a fluid; a detection element which detects an amount of displacement of the diaphragm; a piezoelectric layer which displaces the diaphragm; and a control circuit which, based on the amount of displacement detected by the detection element, applies a drive voltage to the piezoelectric layer which maintains the position of the diaphragm at a prescribed position, and outputs a signal indicating the drive voltage applied to the piezoelectric layer as a signal indicating the pressure of the fluid.

2. The pressure sensor according to claim 1, wherein the piezoelectric layer includes one or more piezoelectric films.

3. The pressure sensor according to claim 1, further comprising: a pair of electrodes which apply the drive voltage to the piezoelectric layer, the diaphragm has a pressure receiving surface which receives the pressure of the fluid, and a reference pressure surface which receives a pressure which is compared with the pressure, the piezoelectric layer and the pair of electrodes are layered on the reference pressure surface, the detection element includes a pressure sensitive capacitor whose electrostatic capacity changes in correspondence with the amount of displacement of the diaphragm, and the pressure sensitive capacitor includes a fixed electrode provided at an opposing portion which opposes the diaphragm in the thickness direction, and the electrode of the pair of electrodes on the opposing portion side is provided as a movable electrode.

4. The pressure sensor according to claim 1, wherein the diaphragm includes sapphire, aluminum nitride (AIN), aluminum oxide (AI2O3), yttrium aluminum garnet (YAG), yttrium oxide (Y2O3), silicon carbide (SiC), magnesium oxide (MgO).

5. The pressure sensor according to claim 1, wherein the material of the diaphragm is the same as or has the same coefficient of thermal expansion as the material of the piezoelectric layer.

6. The pressure sensor according to claim 1, further comprising: a pair of electrodes which apply the drive voltage to the thickness direction of the piezoelectric layer, the piezoelectric layer expands and contracts in a direction perpendicular to the thickness direction when the drive voltage is applied to the thickness direction.

7. The pressure sensor according to claim 1, wherein the detection element includes a pressure sensitive capacitor whose electrostatic capacity changes in correspondence with the amount of displacement of the diaphragm, and a reference capacitor which has a reference capacity which is compared with the electrostatic capacity of the pressure sensitive capacitor, the control circuit detects the amount of displacement based on the electrostatic capacity and the reference capacity.

8. The pressure sensor according to claim 1, further comprising: a pair of electrodes which apply the drive voltage to the piezoelectric layer, the detection element includes a pressure sensitive capacitor whose electrostatic capacity changes in correspondence with the amount of displacement of the diaphragm, one of the pair of electrodes constitutes the pressure sensitive capacitor.

9. The pressure sensor according to claim 1, further comprising: a pair of electrodes which apply the drive voltage to the piezoelectric layer, the detection element includes a pressure sensitive capacitor whose electrostatic capacity changes in correspondence with the amount of displacement of the diaphragm, the two electrodes which constitute the pressure sensitive capacitor are provided independently of the pair of electrodes. ​

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